My guess on the diopter is that you're looking at the wrong thing to adjust the focus. I know I did that multiple times until I figured things out. If the son has 20/20 vision, it's pretty clear that it should be able to adapt properly. The diopters don't have infinite range of adjustment, so for example they cannot correct for my eyes (-6.5 diopters in one of the eyes - I am as blind as a bat) but from what I recall they can usually adjust from about -2 to +1.5 diopters. If you are "a bit" farsighted, you should probably be within the 0-1.5 range (assuming that my memory is correct).
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!